This invention relates to focus-position compensators for microlens arrays. More particularly, it relates to using tiles to affect the focus-position compensation.
Associated with the information revolution is a need to increase by many orders of magnitude the rate of information transfer. This revolution is enabled by the switch from copper wire to optical fiber. Efficient implementation of this change requires optical switches to move data from one fiber to another. For a large number of input and a large number of output fibers, this switch is typically referred to as an optical crossbar switch.
A typical component of an optical crossbar switch is a fiber array coupled to a microlens array in such a way that an array of substantially collimated and parallel beams leave the assembly. A schematic of a microlens array is illustrated in FIG. 1. Each microlens array 100 is comprised of a plurality of lenslets 110. In the typical case, each optical fiber is associated with a single lenslet 110.
A one-to-one mapping exists between fibers and optical beams leaving the assembly. The system performance is enhanced if each optical beam is substantially focused on the end of its respective optical fiber. The construction of such a system is simplified if all of the beams focus through the microlens array at substantially the same distance. In such a case, the ends of all the optical fibers are arranged on a plane that is a uniform distance from the microlens array. This requires that the microlens array have a high degree of uniformity with respect to the distance at which each lenslet focuses.
Manufacturing a microlens array with sufficiently high uniformity with respect to the focus distance is expensive. Most often, the problem is associated with variations in the focal length of the individual lenslets. However, for the purposes of this patent document, variations in the focus or focus distance can be due to focal-length variations of the lenslets or any other source of nonuniformity. In more affordable microlens arrays the focus distance tends to vary slowly across the array. A typical variation is illustrated in FIG. 2. For this particular microlens array, the low regions 120 indicate portions of the microlens array for which the focus distance is as much as 3% less than the nominal value. The high regions 130 indicate portions of the microlens array for which the focus distance is as much as 4% greater than the nominal value.
To reduce the cost of an optical crossbar switch and maintain satisfactory performance, a means for compensating for the microlens focus variations needs to be developed.
Embodiments of the invention include a variety of focus-position compensators for reducing the focus variations of a microlens array. For the purposes of this application, reducing the focus variations is to be interpreted broadly. The reduction in variations can be associated with reduced maximum variations, reduced mean-square variations, reduced root-mean square variations, or some other rational measure of focus variations.
Focus-position compensators of the present invention include a plurality of tiles. Each tile has its index of refraction and its thickness chosen to obtain a specified tile focus-position adjustment or correction. The tiles are disposed in relation to the microlens array such that the effects of focus variations of the microlens array are reduced.
The invention also includes methods for making focus-position compensators for a microlens array. To practice the method, the spatial variation of focus distances of the microlens array is determined. To reduce the spatial variation of the focus distances to within a desired limit, tiles are placed in the light path between the microlens array and optical fibers. The number of tiles, the spatial distribution of tiles, and the tile focus-position corrections are chosen. For each tile, the tile focus-position correction is a function of the tile thickness and the tile refractive index; hence these properties are selected for each tile.
A reference thickness is chosen that is greater than or equal to the maximum of all the tile thicknesses. Spacer-block thicknesses are determined for all the tiles. The spacer-block thickness is equal to the difference between the reference thickness and the tile thickness. The tiles are constructed, each having its specified thickness and refractive index. All spacer blocks with non-zero spacer-block thickness are constructed.
A tile tray having a receptacle for receiving each tile is micromachined. The receptacles are positioned so that when populated with tiles, each tile will be properly situated relative to the other tiles.
The spacer blocks and tiles are placed in their receptacles. If the corresponding spacer block exists (i.e., the spacer block has non-zero thickness) then the tile is placed on top of the spacer block. For tiles that don""t have a corresponding spacer block, the tile is simply placed into its receptacle.
A curable bonding material is placed on top of each tile. An intervening structure is placed on the curable bonding material. The intervening structure can be the substrate of the microlens array, a fiber-block window attached to optical fibers, or a window otherwise disposed between the optical fibers and the microlens array. The bonding material is then cured, securing the tiles to the intervening structure. The tile tray and spacer blocks are then removed.
In lieu of the curable bonding material, adhesive free bonding or fusion bonding may be used to bond the tiles to the intervening structure.
Additional features and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Various embodiments of the invention do not necessarily include all of the stated features or achieve all of the stated advantages.